The overall goal of this project is to elucidate the mechanism of regulation of surfactant secretion. We are particularly interested in how surfactant secretion in the newborn is stimulated by physiological factors such as labor and ventilation. Such information is essential for development of rational therapeutic approaches to prevention of the respiratory distress syndrome of the newborn (RDS) which is still a major cause of illness among premature infants in the United States. Based on data generated during the previous funding period, this renewal application focuses on purinoceptor agonists and leukotrienes as possible mediators of surfactant secretion. The proposed experiments will utilize type II cells isolated from adult and fetal rat lungs as well as the newborn rabbit in vivo. There are four specific aims. 1. To determine how purinoceptor agonists stimulate phosphatidylcholine secretion. 2. To determine how leukotrienes stimulate phosphatidylcholine secretion. 3. To determine if the stimulatory effects of labor and ventilation on surfactant secretion in the newborn rabbit are mediated by purinoceptor agonists and/or leukotrienes. 4. To determine if fetal type II cells also respond to purinoceptor agonists and leukotrienes and if their receptors increase developmentally and/or in response to hormones which are known to influence fetal lung maturation. The first two questions will be addressed in primary cultures of adult type II cells. We will determine which purinergic and leukotriene receptors are involved by examining the effects of a variety of agonists and antagonists on secretion and by measuring specific radioligand binding to membrane receptors. We will address the intracellular mechanisms by which these secretagogues mediate their effects by measuring cAMP levels as well as activities of adenylate cyclase, cAMP-dependent protein kinase and protein kinase C and by comparing their effects with those of other known secretagogues - beta-agonists, phorbol esters, OAG (1-oleoyl-2-acetyl-glycerol) and PAF (platelet activating factor). The third specific aim will be addressed in newborn rabbits in vivo. We will attempt to mimic and block the physiologically induced secretion with specific agonists and antagonists as well as lipoxygenase inhibitors. The final aim will be addressed in type II cells isolated from fetal rat lungs.